I have a 1977 ford 361fe engine with a 4 barrel holley carb that has a built in governor. It is the OE carb. Upon starting a cold engine the truck runs great. After 5 to 10 min of run time the primary flaps do not open upon pressing the throttle. Fuel squirts but the flaps do not open. This is only on a hot engine. I can manually push them open with a screw driver and the engine will run great.They will stay open with the throttle once assisted. There must be some kind of vacuum problem in the carb when warm. Any ideas?